1. Field of the Invention
The present invention relates to a printing apparatus which feeds a printing paper using a paper feeding cassette, and in particular, to a printing apparatus which uses a dam type paper feeding cassette in such a manner that a printing paper is separated and fed using a paper separating guide.
2. Description of the Related Art
Generally, a printing apparatus such as an ink jet printer, laser printer, copy machine, etc., uses a paper feeding apparatus which is capable of continuously printing a plurality of printing papers to implement a continuous printing operation. A dam type paper feeding cassette is used as one type of the continuous paper feeding apparatus. This type of paper feeding cassette uses a paper separating guide. FIG. 1 is a view illustrating an example of a paper feeding cassette for a dam type printing apparatus.
As shown in FIG. 1, the paper feeding cassette includes a cassette 10, a pick-up roller 30, a picking arm 20 and a paper separating guide 40.
As shown in FIG. 2, the cassette 10 stacks a plurality of printing papers 1 therein and is installed in a body (not shown) of the printing apparatus. A pad 11 is installed in a bottom of the cassette 10 to transfer the last printing paper 3.
The pick-up roller 30 is installed in an upper portion of the cassette 10 and pressurizes the printing papers 1 stacked in the cassette 10 and rotates to thereby transfer the printing papers 1 in a direction of the paper separating guide 40.
The picking arm 20 is installed in such a manner that one end 21 thereof is rotatable with respect to the body of the printing apparatus, and the other end is connected to a shaft of the pick-up roller 30. When the height H of the printing papers 1 stacked in the cassette 10 is changed, since the pick-up roller 30 is rotated with respect to the one end 21 of the picking arm 20, the pick-up roller 30 continuously pressurizes the printing papers 1. The picking arm 20 includes a driving force transferring unit (not shown) that receives a driving force from a driving source (not shown), such as a motor installed in the body, and transfers the received driving force to the pick-up roller 30. Therefore, the pick-up roller 30 is rotatable in a state in which an upper surface of the printing papers 1 is pressurized.
The paper separating guide 40 is installed in the forward part of the cassette 10 and is installed in such a manner that the paper separating guide 40 is inclined in a transferring direction of the printing papers 1. A surface 41 of the paper separating guide 40 has a friction coefficient associated therewith. An uppermost printing paper 2 receives the picking force from the pick-up roller 30, which is greater than the friction force, thereby allowing transfer of the paper 2. The printing papers 1, which are placed below the uppermost printing paper 2 and have a picking force which is smaller than the friction force of the paper separating guide 40, are not transferred over the paper separating guide 40.
The operation of the paper feeding apparatus for the conventional printing apparatus will be described with reference to FIGS. 1 and 2.
When a printing operation begins, a driving force is transferred to the pick-up roller 30 through the picking arm 20. At the same time, the picking arm 20 is rotated downward with respect to an engaging portion of the body of the printing apparatus, and the pick-up roller 30 presses on an upper surface of the uppermost sheet of printing paper 2 of the cassette 10. The engaging portion attaches to the end 21 of the picking arm 20.
Thereafter, the uppermost sheet of the printing paper 2 is transferred to the paper separating guide 40 by the friction force and rotation of the pick-up roller 30. At this time, a plurality of printing papers 1 are transferred in the direction of the paper separating guide 40 by the pick-up roller 30. However, only the uppermost printing paper 2 has a transfer force from the pick-up roller 30 which is larger than a friction force applied by the paper separating guide 40. Thus, only the uppermost printing paper 2 is transferred to the body of the printing apparatus via the paper separating guide 40. However, since the transferring forces of the printing papers 1 are smaller than the friction force of the paper separating guide 40, the printing papers 1 do not pass through the paper separating guide 40. Therefore, the printing papers 1 stacked in the cassette 10 are separated one by one, and the separated paper is fed to the body of the printing apparatus.
When the printing papers 1 are continuously transferred, the stacking height H of the printing papers 1 is decreased. In this state, the picking arm 20 is rotated downward, so that the pick-up roller 30 maintains close contact with an upper surface of the printing paper 2. In the above manner, the uppermost printing paper 2 is continuously fed to the body of the printing apparatus through the paper separating guide 40.
However, if thick printing papers are stacked to a maximum height H in the cassette 10 and are then printed, a picking error, in which the printing paper 2 is not picked up by the pick-up roller 30, occurs.
The above picking error occurs when the paper transferring force of the pick-up roller 30, namely, the picking force, is smaller than the friction force of the paper separating guide 40.
The above problem will be described in more detail. As shown in the graph of FIG. 3, the transferring force applied to the printing papers 1, namely, the picking force, changes based on the stacked height H of the printing papers 1. However, the friction force applied to the printing papers 1 by the paper separating guide 40 is determined based on a surface friction coefficient of the paper separating guide 40 and the type of paper used. Therefore, if the types of the printing papers 1 are the same, the friction force is the same irrespective of the stacked height H. Line C in FIG. 3 illustrates a friction force of the paper separating guide 40 applied to a thin paper (for example, 60 g/m2). Line B in FIG. 3 illustrates a friction of the paper separating guide 40 applied to a thick paper (for example, 105 g/m2).
As shown in FIG. 3, in the case of the thick printing paper, in a certain portion (portion X) in which the height of the printing papers 1 stacked in the cassette is high, the friction force of the paper separating guide 49 is larger than the picking force of the pick-up roller 30. In this case, a picking error in which the printing paper is not transferred by the pick-up roller may occur.
In order to overcome the above problems, the surface friction coefficient of the paper separating guide 40 can be decreased. However, if the paper is thin or has a lot of static electricity, the uppermost printing paper 2 is not properly separated, and multiple sheets of the printing papers are fed at once. Therefore, it is difficult to employ the paper separating guide 40 in the printing apparatus in which various types of printing papers are used.